On a planet without axial tilt and with a roughly circular orbit, there would be no seasons. The climate on the planet would be, as far as I can tell, exactly the same at any time of year, and the length of days (and thus the position of the sun in the sky) wouldn't change either.

In a circumstance like that, how might the inhabitants track the passage of years? Would they even be able to? The idea of a year is so fundamental to our understanding of time that it seems odd that it might not be detectable, but I can't really think of any other way for a primitive civilization to measure something as basic as a year.

Are there any primitive (say, pre-Rennaisance) methods for measuring a year's length (i.e. the time it takes the planet to circle its star) that aren't based on axial tilt or an eccentric orbit?

$\begingroup$In our world, are there locales near the equator where there are no seasons? If so, investigate how they do (did) it.$\endgroup$
– GillesOct 15 '14 at 22:58

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$\begingroup$@Gilles They still have seasons at the equator. They're much less significant than those at, say, may latitude here in Alaska, but still enough to be noticed. Or so say the several friends I have who do or have lived at (or very near) the equator.$\endgroup$
– KromeyOct 15 '14 at 23:06

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$\begingroup$"The idea of a year is so fundamental to our understanding of time" It started as our major way to track the movements of migrating herds & flights of birds and when to collect plant materials, then later, when to plant crops. Without seasonal variation in the food supply, why would they bother tracking the year? Or to put that another way, I think a year is so fundamental to us because we have yearly seasons.$\endgroup$
– Andrew ThompsonOct 16 '14 at 5:06

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$\begingroup$Keep in mind that we went through a lot of history without keeping track of the exact date. Historical reports are often in the form of "during the reign of..." Except for astronomers, nobody cared.$\endgroup$
– TwinklesOct 16 '14 at 6:50

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$\begingroup$The year would fall naturally from looking at the stars at night. It's not as obvious to us now with unnatural lighting at night, but it would be immediately clear to a pre-industrial society that even paid passing attention to the stars.$\endgroup$
– John MeachamNov 25 '14 at 2:27

7 Answers
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If you picture yourself looking at a map of your solar system showing the plane of your planet's orbit, imagine that "up" on your map is "north". Put a star up there. When your planet is "north" of its sun, that star would be directly overhead at night. 3 months later (on a 12-month calendar), when your planet is "west", that star is barely visible on the horizon at night. Another 3 months later, it's on the opposite side of the sun at noon. Another 3 months, and it's coming up on the other horizon at night, and one more 3 months puts it directly overhead at night again.

Stars that are more-or-less in line with the plane of a planet's orbit are only visible seasonally, whether you have "proper seasons" or not. Even circumpolar stars, visible year-round, could be observed to "move" in the night sky based on the time of year. With this information alone even very early observers of the night sky would see these seasonal changes and, even in the absence of any other annual cycles, would be able to accurately determine the length of their year and devise calendar systems to measure it -- all you need to measure it is a simple astrolabe, a tool that's been around on Earth since at least 150 BC and which was the basis of some incredibly accurate atronomical observations and studies in the ancient world.

The moon.

If your planet has a moon (or many moons), it will almost certainly go through phases similar to our own moon; while not a "year" per se, if it's like our lunar cycle it's a good basis for a "month". Multiple moons can result in more complex interactions, with epochs based on when they match up again; that is, one moon might have a 27-day cycle, while another has a 31-day cycle, and an epoch could be the time between when both moons have the same phase at the same time (which, in this example, would be 837 days).

Of course, if you're specifically looking for a measurement of the planet's orbit/year, the lunar cycle probably isn't that helpful, except potentially as a basis for the first subdivision (as our months are (very) roughly based on the lunar cycle, and are the first subdivision on our own calendar).

The idea of a year is so fundamental to our understanding of time that it seems odd that it might not be detectable, but I can't really think of any other way for a primitive civilization to measure it.

Why does the civilization have to measure time in the same way we do?

You are making the implicit assumption that measurement through astrological phenomena is the only way to measure time.

On a planet without seasons, how would people track years?

All that is needed is a cyclic, easily measurable, and available. The only key is you need the ability to determine it.

Don't confuse the unit of measure with the measurement tool. The unit of measure on Earth is based on a day (which, as you might note, is actually not totally accurate - there are adjustments made for leap years and lea seconds). But the tools of measurement nearly never use the solar cycle for their input.

Humans used the moon initially but many other methods exist too. Some of the many listed in that article:

Oil lamps

Candle clocks

Water clocks

Hourglasses

Mechanical clocks

None of them necessarily have to correlate to a time derived from phases related to celestial movement. In fact, carbon dating uses a system to determine time passage which could be 100% independent of the sun, moon, or earth.

It is also interesting that even though humans have years, they still adjust it for things such as dog years.

It's entirely possible -- and perhaps likely -- society would have found an additional measurable quantity which could become something similar to what humans deem years.

$\begingroup$That first quote was intended to mean "I can't think of another way to measure a year." This question isn't about measuring time, obviously there are any number of arbitrary ways to measure that. I'm specifically asking about how they might measure their planet's orbit around their sun, aka their year.$\endgroup$
– NerrolkenOct 16 '14 at 0:37

$\begingroup$@Nerrolken I have suggested an edit to your question to clarify your intent.$\endgroup$
– enderlandOct 16 '14 at 0:43

I would say if they don't have distinct years, then they don't need to track them. If there is no difference in weather and temperature then they would never have bothered to clock the non-existent seasons. The wouldn't need to work out when to plant and when to harvest.

Trees and people would have to be sorted by size instead of age - smaller than this, big enough for this.

That would be enough for everyday business. They would still need hourglasses for boiling eggs and so on, and would have to divide the days into groups for tenancy questions and tracking pregnancy and suchlike. The day-groups could be set by number of fingers - 8, 10, 12? Maybe a moon would help them set months.

I suppose scientists and engineers would track the stars and moons, as someone mentioned above, but it would be a purely academic exercise.

$\begingroup$I agree most with this. Time isn't tracked until it becomes useful to track it. Months were tracked initially so change in seasons wouldn't be surprising. Rather than give one's age in years some civilizations gave them in "plantings" or "harvests" - again according to seasons. These developed into years. If there are no seasons, then there's little reason to track years or even months in pre-industrial ages.$\endgroup$
– Adam DavisApr 15 '15 at 16:32

$\begingroup$I have to disagree for the sake of national and religious holidays and because without a year they would be writing the 24,000th new moon for historical records it can be good for organization and essential for holidays.$\endgroup$
– Echo61505Mar 25 at 21:06

On a planet without seasons, people would track years in exactly the same way that people on Earth would track months if we didn't have a moon.

That is, we wouldn't.

The Moon has a very blatant regular cycle. Everyone in every society can easily see it and notice the changes from night to night. The Moon, and its months, became very fundamental to society. A person could agree to meet someone at the second full moon from now, and everyone will know exactly when that is. The phases of the moon will even indicate how soon that meeting will be. Even today, traditional calendars such as Hebrew, Muslim, and Chinese are all synchronized with the Moon's phases.

But in today's society, our calendar months don't even match the Moon's phases. Most people that live in cities can't tell you what the current phase of the Moon is now; most of them couldn't tell you even if they were looking at it. Can you?

On a planet without seasons, the year would be indicated by the regular cycle of the position of the stars in the sky. This would be of significance to astronomers and navigators, but for normal people, it would have as much significance as the current position of Jupiter in the sky is to us here on Earth. Effectively no one knows or cares.

If your planet has a large obvious moon, then your people would measure time in months. If not, and there is some other obvious phenomenon with a regular cycle, they would use that.

But I would say that more significantly, if there weren't seasons, your planet likely wouldn't have any civilizations either.

Without seasons, one day is like the next. If you were sheltered last night and you found enough food and water yesterday, you can reasonably expect to do the same today. And you can reasonably expect to continue that way for the rest of your life.

In such a world, why would anyone bother to develop agriculture or technology? Without change and the need to plan during good times for survival during the bad times, people would simply live their lives day to day.

It's the regular predictability of seasons that allows us to know in Spring that we can plant seeds so that if we tend them in the Summer, in the Fall we will be able to gather enough food to last us through the Winter. It's the benefit of co-operating with others to make this process more efficient that brings people together into societies. It's the ability to visualize larger and better ways of doing it that creates technology.

If your people know that today is just like tomorrow, and next week, and next month, and so on, they will have no need to track time, and their society, if it forms at all, will remain stagnant.

The rotation of the Planet around its star, when fully completed, defines a year in the perspective of the Planet.

Circular Orbit Considerations

If the orbit is highly circular, and not elliptic as in nature (which is caused by the interactions of many masses in a solar systems, combined with the eigenrotation of the celestial body, aka planet and its possible satellites aka moons, in question) there would still have to be set a point in its orbit from which the measuring is started.

Once the planet has reached back to this point, a year has passed.

Tool of Triangulation

If the planet possesses a moon, and the distance gradient of the orbit (the difference of distance between star and planet while planet is moving around star) is sufficient to show up in calculations when triangulated against the distance of the planet to the moon, a year can be determined by mathematicians.

Limitation to determining Year

If the primitive civilization is in a state of pre-Ancient Greece Mathematics, they would indeed be no way for them to tell the time other than resorting to counting Sun Rise and Sunsets.

$\begingroup$Not only does this not answer the question (the question is about how to relate some short clock, like the solar day, to a solar year -- or more importantly, how to even realize the solar year exists and is worth keeping track of), it's also not very accurate. Menstrual cycles are not particularly regular (commonly being off by several days from month to month), and are certainly not as regular as the time from solar noon to the next solar noon.$\endgroup$
– MichaelSOct 13 '16 at 9:16

It would take them a much longer time to accurately describe a year

As another answer already mentions, if it's not useful for the people on your planet, they they probably won't use it.

They can still track stars, and that would most probably develop as the first trading lines (by land or sea) are established. But a year wouldn't be very well defined until much later. Let me explain.

Nonetheless, they would probably just use days, and some other grouping of days resembling our months. If they have a moon, then probably the moon's phases would be use as a group. If they don't, then I assume whatever the maximum number their number system allows without 'rounding' to the next figure (ie: our maximum number with a sole number is 9, so 10. If they have a duodecimal number system, then 12).

They will most likely approximate a year to an integer multiple of those groups first, and then refine from there until they know it precisely.
Then they would refine the groups so that the newly refined year can be divided into somewhat equal length groups, as we did. Although we had it easier, the year was evident to us long before the months. But I would assume you would end up in the exact same spot as us, it would probably take longer though.